The present invention is directed to apparatus and systems for detecting metallic objects of a predetermined type buried in the ground, and particularly to metal detector apparatus which distinguish such predetermined objects from distributed mineral material and undesired objects.
In attempting to locate valuable coins and jewelry, and other metallic objects, such as pipes, metal detector apparatus typically utilize an electronic circuit and search coil system which produces a varying magnetic field and an electric "receive" signal induced by that field, and utilizes the aforementioned electric signal to detect distortion in the magnetic field resulting from the presence of various conductive or magnetic materials or objects. Metal detector systems utilizing such a circuit and search coil arrangement are disclosed in Penland U.S. Pat. No. 3,549,985; Penland U.S. Pat. No. 3,471,773; and Wheeler U.S. Pat. 2,451,596. A particularly useful search coil arrangement is also disclosed in the U.S. patent application of Fredrick H. Thompson, Ser. No. 039,175, filed May 15, 1979.
While it is a relatively straightforward matter to detect some distortion in a magnetic field which indicates the presence of such materials or objects, it is important to distinguish desired "target" objects from unwanted objects so that time is not wasted digging for unwanted objects, and there are often undesirable objects or materials in close proximity with a desired target object. Heretofore it has been difficult to distinguish the target object from those undesirable items effectively. For example, distributed mineral material is present in the ground almost everywhere and causes distortion in the magnetic field produced by such a search coil. At the same time, other items such as aluminum container pull tabs and ferrous bottle caps are also distributed in the ground at various locations, typically in areas where people have gathered, which, unfortunately, is also where sought-after valuable items such as coins are found. Accordingly, an effective metal detector should be capable of distinguishing a desirable object, such as a coin, from distributed mineral material and other undesirable objects.
It is well known that different materials distort a magnetic field in different ways which are characteristic of the material. Consequently the electric signal produced by a search coil in response to the proximity of such materials will be comprised of various signal components corresponding to the various field-distorting objects in close proximity to the coil, each of those components having a generally distinct and characteristic phase relationship with the magnetic field initially generated by the search coil. Such phase relationships can be empirically determined and utilized to identify different items in the proximity of the search coil from one another. For example, systems capable of identifying materials by their phase characteristics are disclosed in Hentschel et al U.S. Pat. No. 3,337,796, which displays relative phase shifts for different materials, Bukner, Jr. U.S. Pat. No. 3,112,443, which discloses an apparatus for recording phase characteristics of different materials, Gardiner U.S. Pat. No. 3,872,380, which discloses apparatus for distinguishing metals based upon their phase characteristics, and Miller et al U.S. Pat No. 3,707,672 and Barringer U.S. Pat. No. 3,105,934, both of which disclose apparatus which analyze the transient response of a signal to identify different materials and objects.
However, it is a more difficult problem to discriminate between various items simultaneously present, particularly desirable target objects, distributed mineral material, and unwanted objects.
One approach for utilizing the difference in phase characteristics produced by various materials simultaneously present to discriminate between them is to sample the receive signal at known times with respect to the transmit signal and analyze the sample signals, the sampling time having a predetermined relationship with the characteristic phase of particular items. An apparatus utilizing this approach has been disclosed in Payne U.S. Pat. No. 4,030,026. The apparatus disclosed in Payne samples the input signal at a time so that the sampled signal excludes any portion due to the reactive component of that signal, thereby distinguishing distortions in the magnetic field due to soil conditions such as distributed mineral material on the one hand, and target items on the other. However, the aforementioned Payne apparatus discriminates only between two predetermined types of materials.
Another approach is disclosed by Payne U.S. Pat. No. 4,128,803. The apparatus disclosed in this patent utilizes not only the characteristic phase differences between items in the ground, but also the characteristic frequency spectrum difference in the electric signal produced by the search coil through motion of the search coil with respect to the objects in the ground. A pair of synchronous demodulators and corresponding low pass filters for the resistive and reactive components of the receive signal, respectively, each eliminate that portion of the signal attributable to distributed mineral materials because of the difference in frequency spectrum, and the outputs of those respective two circuits are utilized to distinguish between a target object and an unwanted object due to the phase difference therebetween. However, such an apparatus requires continuous movement of the search coil relative to the search area, thus inhibiting exact location of target objects.
In general, it is known that one can discriminate between items in the proximity of a metal detector search coil by sampling the receive signal at predetermined times when components of that signal attributable to particular materials are at zero amplitude, and comparing those samples. Heretofore, however, such an approach has resulted in substantially reduced sensitivity due to proximity of the phase between desired and unwanted objects.
Accordingly, it can be seen that there is a need for an improved metal detector system which minimizes interference from soil conditions, that is, mineral materials distributed in the ground, without requiring constant and consistant movement of the search head, permits discrimination between various metallic objects simultaneously in the proximity of the search coil, and minimizes any loss of sensitivity in such discrimination.